Looking at the sky on a clear night, you can be sure of what our ancestors did not even suspect: at least one planet revolves around almost every star.
Worlds in the orbits of other stars are called "exoplanets" and range from giant gas giants larger than Jupiter to small rocky planets like Earth or Mars. Distant planets can be hot enough for metal to melt on their surfaces, or as icy snowballs. Many of them revolve so quickly and closely around their stars that their year lasts several Earth days. Some may have two suns. There are wanderers expelled from their systems, those who wander in the dark across the galaxy.
The Milky Way is a huge family of stars, spanning approximately 100,000 light years. Its spiral structure contains about 400 billion inhabitants, and our Sun is among them. If each of these stars has not one planet in its orbit, but several, as in the solar system, then the number of worlds in the Milky Way is simply astronomical: the count goes to trillions.
The star systems that live in the Milky Way. Credit: ESA / Hubble / ESO / M. Kornmesser.
For several centuries, mankind has been thinking about the possibility of the existence of planets around distant stars, and now we say with confidence that extrasolar worlds do exist. Our nearest neighbor, Proxima Centauri, recently had a rocky planet discovered, and it probably isn't alone. The distance to it is approximately 4.5 light years or 40 trillion kilometers. However, most of the exoplanets found are located hundreds or thousands of light years away.
Bad news: we have no way to get to them yet. The good news is that we can look at them, estimate temperatures, “probe” the atmosphere, and perhaps in the near future we will discover signs of life that are hidden in the dim light coming from these distant worlds.
The first exoplanet to enter the world arena was 51 Pegasi b, a "hot Jupiter" 50 light-years from us that orbits a star in 4 Earth days. The turning point after which extrasolar planets became commonplace came in 1995.
Artistic representation of hot jupiter. Credit: ESO.
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Before 51 Pegasi b, there were several candidates. The exoplanet known today as Tadmor was discovered in 1988. Although its existence was called into question in 1992 due to insufficient evidence, ten years later additional observations confirmed that a planet did orbit around Gamma Cepheus A. Then, in 1992, a system of "pulsar planets" was discovered. These worlds orbit around a dead star, the pulsar PSR 1257 + 12, which lives 2,300 light-years from Earth.
We now live in a universe of exoplanets. Their number is constantly increasing, and at the moment the number of confirmed planets outside the solar system has crossed the line of 3700, but in the next decade, the schedule may jump to tens of thousands.
How did we come to this?
We are on the verge of great discoveries. The era of early exploration and the first confirmed exoplanets set the stage for the next phase: the hunt for distant worlds with more "vigilant" and sophisticated telescopes in space and on earth. Some of them have been tasked with conducting an accurate population census, calculating the various sizes and types of exoplanets. Others scrutinize individual worlds, their atmospheres, and their potential to sustain life.
Direct visualization of exoplanets, that is, their actual images, is playing an increasingly significant role, although scientists have reached the current level of knowledge mainly by indirect means. The two main methods are wobbles and eclipses.
Animation compiled from images of four massive exoplanets orbiting young star HR 8799. Credit: Jason Wang / Christian Marois.
The first is based on fixing distinct oscillations of stars under the influence of gravity of an orbiting planet. These deviations characterize the mass of the exoplanet. The method made it possible to confirm the first candidates, including 51 Pegasi b, and in total, by measuring the radial velocity, about 700 worlds were discovered.
But the vast majority of exoplanets are found by the transit method, which is based on capturing an incredibly tiny drop in the luminosity of a star when a planet crosses its disk. This search strategy indicates the size of the object. NASA's Kepler Space Telescope, launched in 2009, has found about 2,700 confirmed exoplanets this way. He is still discovering new worlds to this day, but, unfortunately, his hunt will soon end, as fuel is running out.
Each method has its own pros and cons. Measuring the radial velocity shows the mass of the planet, but does not provide information about its diameter. The transit speaks of the size of the extrasolar world, but does not allow determining the mass.
But, when several methods are used together, we can obtain important data about the planetary system without direct visualization. The best example is TRAPPIST-1, about 40 light years away, in which seven Earth-sized planets orbit a small red dwarf.
Planets orbiting the ultra-cool red dwarf TRAPPIST-1 compared to Earth. Credit: ESO / M. Kornmesser.
The TRAPPIST-1 family has been studied by ground-based and space telescopes. Research has shown not only the diameters of seven densely packed planets, but also their subtle gravitational interaction on each other. Now we know their masses and diameters, we can estimate the temperature on the surface and even guess the color of the sky on each of them. While much is still unknown about these seven planets, including whether they are covered in oceans or a crust of ice, TRAPPIST-1 has become the most studied star system besides our own.
What's next?
The next step will be a new generation of space telescopes. First of all, TESS, which is scheduled to launch on April 16, 2018. This modern instrument will conduct an almost complete survey of nearby bright stars in search of transiting planets.
TESS will select the best candidates for closer inspection by the James Webb Space Telescope, which will go into space in 2020. The Hubble's successor, with its huge mirror, will collect light directly from the planets themselves, which can then be decomposed into a spectrum, a kind of barcode showing what gases are present in the exoplanet's atmosphere. The main targets of the telescope will be "super-earths".
"Hunter" for exoplanets TESS. Credit: NASA.
Little is known about this class of extrasolar worlds today, including whether they are habitable. The reason for this is the lack of analogues of the super-earth in the solar system. If we're lucky, one of them will show signs of oxygen, carbon dioxide, and methane in its atmosphere. However, the hunt for the atmospheres of Earth-sized planets will have to be postponed until the next generation of space telescopes in the 2030s.
Thanks to the Kepler telescope, we now know that the stars above us are surrounded by planets. And we can be sure not only of a huge variety of exoplanetary neighbors, but also that the adventure is just beginning.
Roman Zakharov